1. Field of the Invention
The present invention relates to a laser diode drive circuit utilized in an optical communication system, and more specifically, a laser diode drive circuit utilized in an optical transmission system that converts an electrical signal to an optical signal by using a laser diode.
2. Description of the Related Art
FIG. 5 shows a laser diode drive circuit 501 in the related art. The laser diode drive circuit 501 comprises drive sources (V+) and (Vxe2x88x92), a switch buffer 503, a switch circuit 507, a laser diode LD, resistors R5 and R505 and the like. The continuity of transistors Q5 and Q6 at the switch circuit 507 is switched by switch control input signals Vin1 (positive phase) and Vin2 (negative phase) to achieve electrical continuity for a drive current IP either with the laser diode LD-side or with the resistor R5-side. The laser diode LD generates an optical signal corresponding to an input signal by emitting/not emitting light. In addition, the level of the drive current Ip which determines the light output power of the laser diode LD is controlled by a base potential Vb of a transistor Q503 which is connected in cascade to the switch circuit 507.
The lower limit of source voltage (V+)xe2x88x92(Vxe2x88x92) at this laser diode drive circuit 501 is expressed as follows:
(V+)xe2x88x92(Vxe2x88x92)xe2x89xa7i Vf+Vsw/2+Vce5+Vce503+Ipxc2x7R505xe2x80x83xe2x80x83(expression 1)
with Vf representing the forward voltage of the laser diode LD, Vsw representing the output amplitude of the switch buffer 503 and Vce5 and Vce503 respectively representing minimum values of the collector-emitter voltages at the transistors Q5 and Q503. More specifically, Vf is approximately 1.5V, Vsw is approximately 0.3V, Vce5 and Vce 503 are both approximately 0.45V, and Ipxc2x7R505 is approximately 0.35V. Thus, the lower limit of the source voltage in the circuit in the related art is calculated by using (expression 1) as; (V+)xe2x88x92(Vxe2x88x92)xe2x89xa72.9V, and a 3.3V single source which is normally used in communication systems at present is utilized for the source voltage.
However, in the circuit structure in the related art achieved by connecting in cascade the current control transistor Q503 to the switch circuit 507, the source voltage lower limit cannot be reduced, and therefore, it cannot support the 2.5V single source expected to become the mainstream source in the future.
An object of the present invention, which has been completed by addressing the problem discussed above, is to achieve low voltage drive and low power consumption in a laser diode drive circuit.
In order to achieve the object described above, the present invention provides a laser diode drive circuit comprising a level shift circuit connected to a drive source, which reduces the drive source voltage by a specific degree, a current switch that turns on/off a laser diode connected to the drive source and a switch buffer connected to the level shift circuit, which implements control on the current switch.